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An Introduction to Ecology
50
BIOLOGICAL SCIENCE
FOURTH EDITION
SCOTT FREEMAN
Lectures by Stephanie Scher Pandolfi
© 2011 Pearson Education, Inc.
Key Concepts
Ecology’s goal is to explain the distribution and abundance of
organisms. It is the branch of biology that provides a scientific
foundation for conservation efforts.
Physical structure—particularly water depth—is the primary
factor that limits the distribution and abundance of aquatic
species. Climate—specifically, both the average value and annual
variation in temperature and in moisture—is the primary factor
that limits the distribution and abundance of terrestrial species.
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Key Concepts
Climate varies with latitude, elevation, and other factors—such as
proximity to oceans and mountains. Climate is changing rapidly
around the globe.
A species’ distribution is constrained by historical and biotic
factors, as well as by abiotic factors such as physical structure and
climate.
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Introduction
• Ecology is the study of how organisms interact with their
environment.
• The central goal of ecology is to understand the distribution and
abundance of organisms.
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Areas of Ecological Study
• To understand why organisms live where they do and in what
numbers, biologists break ecology into several levels of analysis.
• In ecology, researchers work at four main levels:
1. Organisms.
2. Populations.
3. Communities.
4. Ecosystems.
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Organismal Ecology
• Organismal ecologists explore the morphological, physiological,
and behavioral adaptations that allow individual organisms to live
successfully in a particular area.
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Population Ecology
• A population is a group of individuals of the same species that
lives in the same area at the same time.
• Population ecologists focus on how the numbers of individuals in a
population change over time.
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Community Ecology
• A biological community consists of the species that interact with
one another within a particular area.
• Community ecologists study the nature and consequences of the
interactions between species and the consequences of those
interactions.
• The work might concentrate on predation, parasitism, and
competition, or explore how groups of species respond to fires,
floods, and other disturbances.
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Ecosystem Ecology
• An ecosystem consists of all the organisms in a particular region,
along with nonliving, or abiotic, components.
• Ecosystem ecologists study how nutrients and energy move among
and between organisms and the surrounding atmosphere and soil or
water.
• Because humans are affecting energy flows and climate, this work
has direct public policy implications.
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How Do Ecology and Conservation Efforts Interact?
• The four levels of ecological study are synthesized and applied in
conservation biology.
• Conservation biology is the effort to study, preserve, and restore
threatened populations, communities, and ecosystems.
Ecologists study how interactions between organisms and their
environments result in a particular species being found in a
particular area at a particular population size. Conservation
biologists apply these data to preserve species and restore
environments.
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Types of Aquatic Ecosystems
• An organism’s environment has both physical and biological
components.
• The abiotic components include temperature, precipitation, wind,
and sunlight.
• The biotic (living) components consist of other members of the
organism’s own species, as well as individuals of other species.
In freshwater and salt water, three key physical factors affect the
distribution and abundance of organisms: nutrient availability,
water depth, and water movement.
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Nutrient Availability
• Nutrients tend to be washed away in moving water, and fall to the
bottom in still water, and thus are in short supply in many aquatic
ecosystems.
• Nutrient levels are important because they limit growth rates in the
photosynthetic organisms that provide food for other species.
• Ocean upwelling and lake turnover affect nutrient availability by
bringing nutrients from the bottom up to the water surface.
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Ocean Upwelling
• In the oceans, nutrients in the sunlit surface waters are constantly
lost in the form of dead organisms that rain down into the depths. In
certain coastal regions of the world’s oceans, however, nutrients are
brought up to the surface by currents that cause upwellings.
• As the surface water moves away from the coast, it is steadily
replaced by water moving up from the ocean bottom.
• The upwelling water is nutrient rich.
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Lake Turnover
• Each year, glacially formed lakes undergo spring and fall
turnovers.
• In winter and summer, the temperature in these lakes varies from
top to bottom along a gradient called a thermocline.
– In winter, surface water is colder while the water at the bottom
is warmer.
– In the summer, surface water is warmer while the water at the
bottom is colder.
• The surface water in winter and summer is oxygen rich, while the
water at the bottom is nutrient rich.
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Lake Turnover
• The oxygen-rich surface water either warms (in spring) or cools (in
fall) to 4ºC—the temperature at which water is most dense—and
sinks to the bottom, carrying oxygen down to the bottom and
driving nutrients up to the surface.
• Without the spring and fall turnovers, most freshwater nutrients
would remain on the bottom of lakes. These aquatic ecosystems
would be much less productive, as a result.
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Water Flow
• The rate of water movement and water depth are the key physical
factors that shape the environments in aquatic ecosystems.
• Water movement is a critical factor in aquatic ecosystems because
it presents a physical challenge. It can literally sweep organisms off
their feet.
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Water Depth
• Water depth dictates how much light reaches the organisms that
live in a particular region.
• Water absorbs and scatters light, so the amount and types of
wavelengths available to organisms change dramatically as water
depth increases, as does light intensity.
• Light has a major influence on productivity—the total amount of
carbon fixed by photosynthesis per unit area per year.
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Freshwater Environments: Lakes and Ponds
• Lakes and ponds are distinguished by size—ponds are small; lakes
are large enough that the water in them can be mixed by wind and
wave action.
• Lakes and ponds have five zones of water depth.
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Water Depth Zones in Freshwater Environments
1. The littoral (“seashore”) zone consists of the shallow waters
along the shore, where flowering plants are rooted.
2. The limnetic (“lake”) zone is offshore and comprises water that
receives enough light to support photosynthesis.
3. The benthic (“depths”) zone is made up of the bottom, or
substrate.
4. Regions of the littoral, limnetic, and benthic zones that receive
sunlight are part of the photic zone.
5. Portions of the lake or pond that do not receive sunlight make up
the aphotic zone.
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Freshwater Environments: Lakes and Ponds
• Water movement in lakes and ponds is driven by wind and changes
in temperature.
• Cyanobacteria, algae, and other microscopic organisms,
collectively called plankton, live in the photic zone, as do the fish
and small crustaceans that eat them.
• Animals that consume dead organic matter, or detritus, are found
in the benthic zone.
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Freshwater Environments: Wetlands
• Wetlands are shallow-water habitats where the soil is saturated
with water for at least part of the year, and contain indicator plants
that only grow in saturate soils.
• Wetlands are distinct from lakes and ponds because they have only
shallow water, and they have emergent plants that grow above the
surface of the water.
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Freshwater Environments: Wetlands
• Wetland types are distinguished by water flow and vegetation.
– Bogs have low or nonexistent water flow, and are stagnant,
acidic, and nonproductive.
– Freshwater marshes and swamps have a slow but steady flow
of water and are relatively nutrient rich and highly productive.
– Marshes have nonwoody plants.
– Swamps are dominated by trees and shrubs.
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Freshwater Environments: Streams
• Streams move constantly in one direction. Creeks are small
streams; rivers are large.
• Most streams are shallow enough that sunlight reaches the bottom.
• The structure of a typical stream varies along its length.
– Where it originates, it tends to be cold, narrow, and fast; at the
end, it tends to be warmer, wider, and slower.
• Streams thus tend to have fewer organism types near their source
(mostly animals) and more varied types near their end (algae,
plants, and animals).
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Freshwater/Marine Environments: Estuaries
• An estuary forms where a river meets the ocean and freshwater
mixes with salt water.
• An estuary includes slightly saline marshes as well as the body of
water that moves in and out of these environments.
• Salinity varies with changes in river flows and with proximity to
the ocean.
• Salinity has dramatic effects on osmosis and water balance; species
that live in estuaries have adaptations that allow them to cope with
variations in salinity.
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Freshwater/Marine Environments: Estuaries
• Most estuaries are relatively shallow, but water depth may vary
dramatically.
• Water flow fluctuates daily and seasonally due to tides, storms, and
floods. This fluctuation alters salinity.
• Species that live in estuaries have adaptations that allow them to
cope with variations in salinity. Estuaries are among the most
productive environments.
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Marine Environments: The Oceans
• The world’s oceans form a continuous body of salt water. Regions
within an ocean can vary markedly in their physical characteristics.
• In terms of water depth, the ocean has six regions:
1. The intertidal zone consists of a beach that is exposed to the
air at low tide but submerged at high tide.
2. The neritic zone extends from the intertidal zone to depths of
about 200 m. Its outermost edge is defined by the end of the
continental shelf—the gently sloping, submerged portion of a
continental plate.
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Marine Environments: The Oceans
3. The oceanic zone is the “open ocean”—the deepwater region
beyond the continental shelf.
4. The bottom of the ocean is the benthic zone.
5. The intertidal and sunlit regions of the neritic, oceanic, and
benthic zones make up a photic zone.
6. Areas that do not receive sunlight are in an aphotic zone.
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Marine Environments: The Oceans
• Water movement in the ocean is dominated by different processes
at different depths.
• Each zone in the ocean is populated by distinct species that are
adapted to the physical conditions present.
• The intertidal and neritic zones are the most productive; the neritic
zone includes coral reefs, which are among the most productive
environments on Earth.
• The photic and aphotic zones are not as productive.
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Types of Terrestrial Ecosystems
• Biomes are major groupings of plant and animal communities
defined by a dominant vegetation type.
• Each biome is associated with a distinctive set of abiotic conditions.
• The type of biome present in a terrestrial region depends on
climate—the prevailing, long-term weather conditions found in an
area.
– Weather consists of specific short-term atmospheric
conditions.
• Climate and weather consist of temperature, moisture, sunlight,
and wind.
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Types of Terrestrial Ecosystems
• The nature of the biome that develops in a particular region is
governed by:
– The average annual temperature and precipitation.
– The annual variation in temperature and precipitation.
• Temperature and moisture influence net primary productivity
(NPP)—the total amount of carbon that is fixed per year minus the
amount of fixed carbon oxidized during cellular respiration.
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Types of Terrestrial Ecosystems
• NPP represents the organic matter that is available as food for other
organisms.
• In terrestrial environments NPP is often estimated by measuring
aboveground biomass, the total mass of living plants, excluding
roots.
• On land, photosynthesis and plant growth are maximized when
temperatures are warm and conditions are wet; conversely,
photosynthesis cannot occur efficiently at low temperatures or
under drought stress.
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Terrestrial Biomes: Tropical Wet Forests
• Tropical wet forests, or rain forests, are found in equatorial
regions where temperatures and rainfall are high and annual
temperature variation is very low.
• The favorable year-round growing conditions produce very
abundant plant growth, leading to extremely high aboveground
biomass.
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Terrestrial Biomes: Tropical Wet Forests
• Tropical wet forests are renowned for their species diversity. The
diversity of plant sizes and growth forms produces extraordinary
structural diversity.
• A tree canopy (the uppermost layers of branches) is intermingled
with vines, epiphytes (plants that grow entirely on other plants),
shrubs, and herbs.
• The diversity of plant growth forms presents a wide array of habitat
types for animals.
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Terrestrial Biomes: Subtropical Deserts
• Subtropical deserts, found at 30º N and 30º S latitudes, are
characterized by high average annual temperatures, moderate
variation in temperature, and very low precipitation.
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Terrestrial Biomes: Subtropical Deserts
• Because the scarcity of water means conditions are rarely favorable
enough to support photosynthesis, the productivity of desert
communities is very low. Plants are widely spaced, perhaps because
of competition for water.
• Desert species adapt to the extreme temperatures and aridity by
growing at a low rate year-round or by breaking dormancy and
growing rapidly in response to any rainfall.
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Terrestrial Biomes: Temperate Grasslands
• Temperate regions have moderate temperatures relative to the
tropics and polar regions. Summers are typically long and warm;
winters, short and cold.
• Temperate grasslands form in temperate regions with relatively low
precipitation.
• This temperature variation dictates a well-defined growing season.
• In the temperate zone, plant growth is possible only in spring,
summer, and fall months when moisture and warmth are adequate.
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Terrestrial Biomes: Temperate Grasslands
• Grasses are the dominant life-form in temperate grasslands because
either conditions are too dry to enable tree growth or encroaching
trees are burned out by fires.
• Although the productivity of temperate grasslands is generally
lower than that of forest communities, grassland soils are often
highly fertile.
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Terrestrial Biomes: Temperate Forests
• In temperate areas with relatively high precipitation, grasslands
give way to temperate forests.
• Temperate forests experience a period in which mean monthly
temperatures fall below freezing and plant growth stops.
• Compared with grassland climates, precipitation is moderately high
and relatively constant throughout the year.
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Terrestrial Biomes: Temperate Forests
• These forests are dominated by deciduous trees.
• Most temperate forests have productivity levels that are lower than
those of tropical forests but higher than those of deserts and
grasslands. The level of diversity is also moderate.
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Terrestrial Biomes: Boreal Forests
• The boreal forest, or taiga, forms on subarctic lands just south of
the Arctic Circle.
• The climate is characterized by very cold winters and short, cool
summers. Temperature variation is extreme.
• Annual precipitation is low, but temperatures are so cold that
evaporation is minimal. As a result, moisture is usually abundant
enough to support tree growth.
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Terrestrial Biomes: Boreal Forests
• Boreal forests are dominated by highly cold-tolerant conifers.
• The productivity of boreal forests is low, but aboveground biomass
is high because slow-growing tree species may be long-lived and
gradually accumulate large standing biomass.
• Boreal forests also have exceptionally low species diversity.
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Terrestrial Biomes: Arctic Tundra
• The arctic tundra is found throughout the arctic regions where land
is not covered in ice.
• Tundra has very low temperatures with high annual temperature
variation and very low annual precipitation.
• The growing season is 6–8 weeks at most; temperatures are below
freezing the rest of the year.
• Most tundra soils are in the perennially frozen state known as
permafrost.
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Terrestrial Biomes: Arctic Tundra
• The arctic tundra is dominated by small woody shrubs, lichens, and
herbaceous plants.
• Arctic tundra has low plant species diversity, low productivity, and
low aboveground biomass.
• Animal diversity also tends to be low.
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Climate and the Consequences of Climate Change
• Each type of aquatic environment and terrestrial biome hosts
species that are adapted to the abiotic conditions present at the
location.
• Climate change is having a profound impact on these abiotic
factors.
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Global Patterns in Climate
• Why does climate vary around the globe?
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Why Are the Tropics Wet?
• Areas along the equator receive the most moisture; locations at
about 30º N and 30º S latitude are among the driest on Earth.
• A major cycle in global air circulation, called a Hadley cell, is
responsible for this pattern:
– Air heated by the strong sunlight along the equator expands and
rises. Warm air can hold a great deal of moisture, because
warm water molecules tend to stay in vapor form instead of
condensing into droplets.
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Why Are the Tropics Wet?
– As the air rises, it radiates heat to space and begins to cool. It
also expands into the larger volume of the upper atmosphere,
which lowers its density and temperature—a phenomenon
known as adiabatic cooling.
– As it cools, its ability to hold water declines, water condenses,
and high levels of precipitation occur along the equator.
– As more air is heated along the equator, the cooler, “older” air
above Earth’s surface is pushed poleward.
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Why Are the Tropics Wet?
– As the air mass cools, its density increases and it begins to sink.
– As it sinks, it absorbs more and more solar radiation from
Earth’s surface and begins to warm, gaining water-holding
capacity. This results in very little precipitation at 30º N and
30º S latitude.
• There are three such circulation cells in both Northern and Southern
Hemispheres.
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Why Are the Tropics Warm and the Poles Cold?
• Areas of the world are warm if they receive a large amount of
sunlight per unit area; they are cold if they receive a small amount
of sunlight per unit area.
• Earth’s spherical shape dictates that regions at or near the equator
receive more sunlight per unit area than regions that are closer to
the poles, due to the angle at which the solar radiation hits the
Earth.
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What Causes Seasonality in Weather?
• Seasons—regular, annual fluctuations in temperature, precipitation,
or both—result from Earth’s 23.5º tilt on its axis.
• In June, the Northern Hemisphere is tilted toward the Sun, so it
faces the Sun most directly and receives the largest amount of solar
radiation per unit area.
• In December, the Southern Hemisphere is tilted toward the Sun,
faces the Sun most directly, and receives the largest amount of solar
radiation per unit area.
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What Causes Seasonality in Weather?
• Thus, summer is in June in the Northern Hemisphere and is in
December in the Southern Hemisphere.
• In March and September, the equator faces the Sun most directly,
so the tropics receive the most solar radiation.
• If Earth did not tilt on its axis, there would be no seasons.
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Physical Features Have Regional Effects on Climate
The broad patterns of climate that are dictated by global heating
patterns, Hadley cells, and seasonality are overlain by regional
effects. The most important of these are due to the presence of
mountain ranges and proximity to an ocean. Mountains and
oceans cause regional effects on climate.
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Physical Features Have Regional Effects on Climate
• The presence of mountain ranges tends to produce extremes in
precipitation.
– In the rain shadow effect, winds from the ocean cool and drop
precipitation on one side of a mountain range and not on the
other side, creating high deserts.
• While the presence of mountain ranges tends to produce extremes
in precipitation, the presence of an ocean has a moderating
influence on temperature because water has a very high specific
heat, or capacity for storing energy.
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Tropical Atmospheric Circulation
Web Activity: Tropical Atmospheric Circulation
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How Will Global Climate Change Affect Ecosystems?
• Climate has a dramatic effect on both terrestrial and aquatic
ecosystems.
• It is now well established that CO2 pollution is causing climate
change. Biologists use three tools to predict how global warming
will affect aquatic and terrestrial ecosystems:
1. Simulation studies are based on computer models of weather
patterns in local regions.
2. Observational studies are based on long-term monitoring at
fixed sites around the globe.
3. Experiments are designed to simulate changed climate
conditions and to record responses by the organisms present.
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Experiments That Manipulate Temperature
• Experimental increases in temperature have been conducted in the
arctic tundra.
• Results show that overall, species diversity decreases, and that
compared with control plots, grasses and shrubs increase inside the
chambers, and mosses and lichens decrease.
• These results support simulation and observational studies
predicting that arctic tundra environments are giving way to boreal
forests.
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Experiments That Manipulate Precipitation
• Simulation and observational studies indicate that global warming
is increasing variability in temperature and precipitation; in other
words, it is making climates more extreme.
• Experimental increases in rainfall variability in temperate
grasslands indicate that average soil moisture and overall NPP
decline with increased variability, as does species diversity.
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Why Are Organisms Found Where They Are?
• Biogeography is the study of how organisms are distributed
geographically.
• Interactions with the abiotic and biotic environments affect where a
particular species lives.
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Abiotic Factors
• The range, or geographical distribution, of every species on Earth
is limited―no organism can live everywhere.
• Because of fitness trade-offs, organisms tend to be adapted to a
limited set of physical conditions—to a particular temperature and
moisture regime on land, or a particular water depth and movement
regime in water.
To understand a species’ distribution thoroughly, it is essential to
examine historical and biotic factors, as well as the physical
conditions present.
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The Role of History
• The first factor to consider in species distribution is the history of
dispersal.
• Dispersal is the movement of an individual from its place of birth
to the location where it lives and breeds as an adult.
• If a species is not found in a particular area, a physical barrier to its
dispersal may exist.
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The Wallace Line: Barriers to Dispersal
• While working in the Malay archipelago, Wallace realized that the
plants and animals native to the more northern and western islands
were radically different from the species found on the more
southern and eastern islands, despite being part of the same biome.
• This biogeographical demarcation is known as the Wallace line; it
separates species with Asian and Australian affinities. A deep
ocean trench maintained a water barrier to dispersal, resulting in
landforms on either side of the line remaining isolated.
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Humans as Dispersal Agents
• In many cases, human activity circumvents physical barriers.
• For example, new flu strains disperse all over the planet in a matter
of weeks or days, transported in the respiratory passages of infected
airplane passengers.
• Humans have also transported thousands of plants, birds, insects,
and other species across physical barriers to new locations.
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Exotic and Invasive Species
• If an exotic species is introduced into a new area, spreads rapidly,
and eliminates native species, it is considered an invasive species.
• In North America alone, dozens of invasive species, such as kudzu,
have had devastating effects on native plants and animals.
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Biotic Factors
• The distribution of a species is often limited by biotic factors—
interactions with other organisms.
• Interactions including competition, reproductive requirements, and
parasitism can all affect the distribution of a species.
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Biotic and Abiotic Factors Interact
• It is often difficult for ecologists to separate the effects of biotic and
abiotic factors on species’ range.
• For example, cheatgrass does not grow in wet grasslands because it
does not compete well against the tall species that thrive there.
• It has, however, been able to invade two important types of biomes
in North America: the arid, shrub-dominated habitats known as
sage-steppe, and dry temperate grasslands.
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The Role of Fire in Cheatgrass’s Spread
• Cheatgrass has been able to invade sage-steppe habitats with the aid
of fire, an abiotic factor that can kill the shrubs that dominate sagesteppe.
• If a fire gets started, it does not affect the cheatgrass because
cheatgrass is an annual, so there is no living tissue exposed once the
growing season is over and because cheatgrass seeds sprout readily
in soils that have been depleted of organic matter by fire.
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The Role of Overgrazing in Cheatgrass’s Spread
• Grazing cattle depleted the bunchgrass populations of the
grasslands in arid areas of the American West. They also
compacted and disrupted the soil.
• Cheatgrass arrived and was able to outcompete re-emerging
bunchgrass.
• Like cheatgrass, the range of every species on Earth is limited by a
combination of historical, abiotic, and biotic factors.
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